1. Field of the Invention
The present invention relates to thread-tapping screw having a shaft having a free end region, a thread extending starting from the screw shaft free end, at least along a portion of the shaft length, and a plurality of cutting bodies formed of a material having a greater hardness than hardness of the thread, located in recesses formed in the thread and opening radially outwardly and having, respectively, a radial outer profile projecting, at least partially, beyond the thread.
2. Description of the Prior Art
Screws which are described above, are screwed directly in a borehole that was preliminary formed in a constructional component, with the thread of the screw forming or tapping a counter-thread or an undercut in the constructional component.
For outside applications, the screw, such as concrete screws, advantageously, are formed of a corrosion-resistant steel. However, steel materials often have a hardness that is not sufficient for tapping a counter-thread in a mineral constructional component, and the thread of the screw wears off during the screw being screwed in. Toughening of the screw or, e.g., of the thread, which is formed integrally with the shaft, is possible only to a limited extent when a corrosion-resistant material is used for forming the screw.
German Publication DE 198 52 338 A1 discloses a screw which is formed of a stainless steel and the shaft and thread of which are provided with a plurality of receiving bores which open radially outwardly and in which cutting bodies in form of cylindrical cutting pin inserts of a hardened steel are received. The cutting pin inserts facilitate cutting-in in hard mineral constructional components such as, e.g. concrete or brickwork. The cutting pin inserts are retained in the receiving recesses in the shaft formlockingly in the screw-in direction and frictionally in the radial direction.
The drawback of the self-tapping screw of DE 198 52 338 A1 consists in that all of the cutting pin inserts project beyond the cross-sectional projection surface of the thread by the same amount. Because the first cutting pin insert, adjacent to the thread pilot, already produces the entire undercut volume for the screw thread, high forces act on this cutting pin insert, this cutting pin insert has a tendency to break off the thread. Further, a high input torque is required for setting such thread-tapping screws, in particular, when the screws have a small diameter.
Accordingly, an object of the present invention is to provide a thread-tapping screw that facilitates setting of the screw in hard constructional component such as, e.g., concrete.
Another object of the present invention is to provide a thread-tapping screw for hard constructional components that can easily be manufactured.